Digestion of cellulose by stomach homogenates of green abalone (Haliotis fulgens)

Abstract

Three experiments were conducted to study the cellulase activity of stomach homogenates of green abalone, with and without antibiotics added. Forty-eight previously frozen stomachs from wild abalone adults were thawed and individually homogenized. Four cultured abalone adults maintained under a balanced diet containing cellulose were killed and the fresh stomach samples processed. Cellulase activity was estimated through the production of glucose, incubating the homogenate from each organism with cellulose. To determine the effect of bacteria, each stomach was incubated (at 25 degreesC for 20 h) in a completely randomized design, with and without an antibiotic mixture. For a time series measurement (exp. 2), four homogenates from wild abalone were used

the incubation mixture was followed up taking samples at 0, 1, 2,4, 8, 16, 20, 32, 64 and 72 h. The latter samples were also used to measure the enzymatic activity before and after a 72-h incubation, in the absence of antibiotics (exp. 3). Initial bacterial counts in the wild abalone homogenates were 459 colonies per mL. In the presence of the antibiotic mixture, no bacterial growth was observed. Initial cellulase activity was 1.6 Units. Cellulose degradation figures were low, even in the absence of antibiotics, indicating the low dependence of abalone on the nutrients derived from the digestion of the cell walls. Still, the addition of antibiotics depressed the digestion of cellulose (14.5 vs. 10.2%), suggesting the importance of live bacteria in the digestion process, and that the majority of the degraded cellulose is perhaps acted upon by endogenous enzymes. The time series experiment of samples from wild abalone, with and without antibiotics, showed quadratic effects on cellulose digestion y = 7.55 + .29h - .005 h(2) (r(2) = .2212), but no significant differences between treatments. The gradual increase in the digestion of cellulose, followed by a plateau is similar to what happens in ruminant digestion. The initial and final enzyme activity without antibiotics showed a significant reduction of 41.1% for the wild abalone whereas in cultured abalone a 50.9% significant reduction was observed. It seems that the cellulase complex enzymes present in the homogenates remain active after 72 h of incubation, even though their activity is lowered by about one half. Although the use of antibiotics assure the absence of live bacteria in the incubation mixture, the cellular enzymes possibly freed by the effect of the antibiotics could still be active for up to 72 h after the microbes are no longer viable. Thus the enzymatic activity attributed to the stomach itself could still be confounded.